My experience with Humbrol enamal is that once you've mixed it as much as humanly possible, and then some more, it's okay. I also agree with the tip about storing the tinlets upside down.
Having said that, I use most water based acrylics these days, if only because they don't affect my asthma. :-)
Another possible tip here people: recently, as an outcome of some indolence inspired experimentation, I made the following discvovery. I had an acrylic "wash" that had settled out. Not wanting to spend ages stirring and / or shaking it, I thought, "what'll happen if I pop it into my ultrasonic cleaning bath for ten minutes, and go do something else?" The result was warm and beautifully re-suspended paint! :-) I've NOT tried it yet with enamals, but will give it a go the next time I have to "restore" some enamal paint before use. Perhaps this could just be the lazy man's answer to paint stirring in future?